In recent years, regulations against chemicals harmful to the environment have got stricter and stricter. Solder materials to be used for bonding electronic components to substrates are not an exception to such regulations. While lead has been used as a main component of solder materials through the ages, it has already been designated as a regulated substance by RoHS directive and similar. Therefore, a solder material that does not contain lead (Pb) (hereinafter referred to as lead-free solder material or unleaded solder material) is under active development.
Solder materials to be used in bonding electronic components to substrates are roughly classified, by limit temperature for use, into a high temperature type (about 260° C.˜400° C.) and a medium-to-low temperature type (about 140° C.˜230° C.). Of the high temperature type and the medium-to-low temperature type, regarding the medium-to-low temperature type solder materials, lead-free solder materials containing Sn as a main component have been put into practical use.
For example, as a lead-free solder material for medium-to-low temperature use, Japanese Patent KOKAI No. 11-77366 listed as Patent Document 1 discloses an unleaded solder alloy composition containing Sn as a main component, Ag of 1.0˜4.0 wt %, Cu of 2.0 wt % or less, Ni of 1.0 wt % or less, and P of 0.2 wt % or less.
On the other hand, with respect to Pb-free solder materials for high temperature use also, various organizations are engaged in development. For example, Japanese Patent KOKAI No. 2002-160089 listed as Patent Document 2 discloses a hermetic seal terminal using, as a solder material, a Bi—Ag alloy film containing 30˜80 mass % Bi.
As Au-based, Pb-free solder materials for high-temperature use, Au—Sn alloys and Au—Ge alloys have already been used in crystal devices, SAW filters, and mount devices for electronic components such as MEMS.
As an Au-based Pb-free solder material for high-temperature use that has been put to practical use, there is counted Au-20 mass % Sn (this means that it is composed of 80 mass % Au and 20 mass % Sn; the same notation applies hereinafter also). It has a eutectic composition and has a melting point at 280° C.
Au-12.5 mass % Ge, which also has been put into practical use, has a eutectic composition and has a melting point at 356° C.
Au—Sn alloys and Au—Ge alloys are differently used in accordance with the difference in melting point. A mount device using a high-temperature type solder material is required to have a heat resistance of 260° C. at lowest. Therefore, Au—Sn alloys having a melting point at 280° C. are mainly used for mount devices requiring heat resistance about 260° C. In a case where a heat resistance higher than 280° C. is required, an Au—Ge alloy is used.
Owing to the melting point property and good reflow wettability, Au—Sn alloys are used for sealing quartz crystal devices, which, in particular, are required to have high reliability. However, Au-based alloys are much harder than Pb-based solder alloys or Sn-based solder alloys, and are very difficult to process into a sheet form. Accordingly, productivity and yield are rendered poor, to raise the cost. In addition, Au-based alloys require extremely high raw material costs as compared with Pb-based solder alloys or Sn-based solder alloys. Thus, more inexpensive products are needed. For the purpose of making the Au—Sn alloys inexpensive and highly usable, there has been developed, for example, an Au-based solder material as shown in Patent Document 3.
Japanese Patent KOKAI No. 2008-155221 listed as Patent Document 3 discloses a brazing filler material having a composition ratio (Au (wt %), Ag (wt %), Sn (wt %)) residing within a region surrounded by the points A1 to A5 below on a ternary composition diagram of Au, Ag and Sn:                Point A1 (41.8, 7.6, 50.5),        Point A2 (62.6, 3.4, 34.0),        Point A3 (75.7, 3.2, 21.1),        Point A4 (53.6, 22.1, 24.3),        Point A5 (30.3, 33.2, 36.6).        
Its object is to provide a brazing filler material and a piezoelectric device that have a relatively low melting point and thus are easy to handle, are excellent in strength and adhesion and are inexpensive.